Abstract Energy transfer between Pr 3 + and Eu 3 + is studied by luminescence spectroscopy and time-resolved measurements. In earlier experiments, Zachau et al. (Proceedings—Electrochemical Society, 97 (29) (Luminescent Materials, pp. 314–324, 1998)) observed no energy transfer from the 1 S 0 level of Pr 3 + to the 5 D 3 level of Eu 3 + for YF 3 : Pr , Eu, in spite of the favorable spectral overlap for energy transfer. This unexpected result was not explained. To resolve this issue, we first calculate the critical distance for energy transfer using the Forster theory and show that efficient energy transfer between Pr 3 + and nearest Eu 3 + neighbors is predicted. Nevertheless, luminescence experiments show no Eu 3 + emission upon excitation in the 1 S 0 level of Pr 3 + in YF 3 : Pr , Eu confirming the results in (Proceedings—Electrochemical Society, 97 (29) (Luminescent Materials, pp. 314–324, 1998)). The experiments do show a strong quenching of the 1 S 0 emission by Eu 3 + . The quenching is explained by a low-energy metal-to-metal charge transfer state ( Pr 4 + – Eu 2 + ) for Pr–Eu pairs. Additional support for this quenching mechanism is provided by the observation that addition of Yb 3 + also quenches the 1 S 0 emission.